The Metropolitan police is considering the purchase of three water cannon at a cost of nearly £4m to cover London and the south east as part of a new approach to public order policing in the aftermath of the summer riots.

If the Met decides to go ahead it will be the first time that water cannon will be a routine option for police outside of Northern Ireland. The development comes as the Met also reveals that more officers are being trained to support its baton round teams, so that plastic bullets can be deployed more "spontaneously" when necessary in fast moving public order situations.

In what appears to be a toughening up of its tactics in the aftermath of the August riots, the force is examining whether further legislation is required to give officers more powers when dealing with large scale disorder as well.

Last week the Met commissioner was challenged over an apparent hardening of the approach when he appeared before the Metropolitan Police Authority. Bernard Hogan-Howe denied that there had been any policy change in relation to baton rounds or water cannon. But the interim report into the riots from the assistant commissioner Lynne Owens, which was published on Wednesday, identifies these two areas as ones where consideration is being given to a new approach following criticism over the summer that the Met had lost control of the streets.

Owens said the Met has explored "in detail" using water cannon as a policing tactic in London – even down to identifying how many are needed and how much they would cost. Having taken evidence from the situation in Northern Ireland the Met believes it would need three water cannons, at a cost of £1.3m each. Scotland Yard is exploring an option with the Association of Chief Police Officers to purchase the equipment as a regional asset for London and the south east and announce a decision next month.

During the riots, serious consideration was given to deploying baton rounds, but these were not used because of what the report said was the "agility" of the disorder and the availability of other tactics. Owens's report shows that the Met has already taken steps to make sure that in future baton rounds could be used in such fast-moving public order situations.

Additional officers are now being trained to work alongside those firearms units who are trained to fire plastic bullets, known as Kestral teams, and making them available to borough commanders.

Owens said: "This increase in capacity will enable the MPS to make more agile use of this tactic in the future. Further expansion has also been explored in answer to the review's findings that a more localised availability of this resource might have enabled commanders to deploy baton rounds as an effective tactic.

"The MPS is also considering the establishment of this tactic as a spontaneous response, thereby making it more readily available across London at short notice."

Owens added that the Met was "fully aware" that active community engagement and consultation would be required before the introduction of any new tactics.

Isabella Sankey, director of policy for Liberty questioned the approach. "It's easy to talk tough in an attempt to send signals about future riots but in practice would water cannon really help?

"It's an indiscriminate tool capable of hurting innocent bystanders and just disperses troublemakers from one area to another."

The Met said it was liaising with HM Inspectorate of Constabulary, before implementing any significant change. One of the greatest challenges the force has identified is intelligence gathering during public order situations, when those involved are using social media to organise and move from place to place.

"Intelligence gathering systems could not cope with the scale and speed and the spread of disorder," said Owens. "The sheer scale of the information flows, communication requirements and co-ordination of resources posed immense challenges."

In response the Met is developing a specialist team of officers who will focus on gathering intelligence and running any investigations which emerge out of large public order events.

The use of social media by police will be "professionalised", the report said.

The Owens report confirms what senior officers have already said about their approach to the rioting – not enough officers were on the scene quickly enough. "The numbers were not enough and they did not arrive quickly enough to deal with the speed with which the violence escalated and its spread," she concluded.